Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography. Flexographic printing plates are relief plates with image elements raised above open areas. Generally, the plate is somewhat soft, and flexible enough to wrap around a printing cylinder, and durable enough to print over a million copies. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made.
In flexographic printing, ink is transferred from a pool of ink to a substrate by way of a printing plate. The surface of the plate is shaped so that the image to be printed appears in relief, in the same way that rubber stamps are cut so as to have the printed image appear in relief on the surface of the rubber. Typically, the plate is mounted on a cylinder, and the cylinder rotates at a high speed such that the raised surface of the printing plate contacts a pool of ink, is slightly wetted by the ink, then exits the ink pool and contacts a substrate material, thereby transferring ink from the raised surface of the plate to the substrate material to form a printed substrate. Those involved in the flexographic printing industry are constantly striving to improve the flexographic printing process in order to more effectively compete.
The demands placed on flexographic printing plates are numerous. Firstly, a flexographic printing plate must have sufficient flexibility to wrap around a printing cylinder, yet be strong enough to withstand the rigors experienced during typical printing processes. Furthermore, the printing plate should possess a low hardness to facilitate ink transfer during printing. In addition, it is important that the surface of the printing plate remains dimensionally stable during storage.
A typical flexographic printing plate as delivered by its manufacturer is a multilayered article made of a backing (or support) layer; one or more unexposed photocurable layers; a protective layer or slip film; and often a protective cover sheet.
The backing layer lends support to the plate and can be formed from a transparent or opaque material such as paper, cellulose film, plastic, or metal. The photopolymer layer(s) can include any of the known binders (oligomers), monomers, initiators, reactive or non-reactive diluents, fillers, and dyes. The term “photocurable” or “photopolymer” refers to a composition which undergoes polymerization, cross-linking, or any other curing or hardening reaction in response to actinic radiation with the result that the unexposed portions of the material can be selectively separated and removed from the exposed (cured) portions to form a three-dimensional or relief pattern of cured material. Preferred photopolymer materials include an elastomeric compound (binder), an ethylenically unsaturated compound having at least one terminal ethylene group, and a photoinitiator. More than one photocurable layer may also be used.
The photopolymer materials generally cross-link (cure) and harden through radical polymerization in at least some actinic wavelength region. As used herein, actinic radiation is radiation capable of effecting a chemical change in an exposed moiety. Actinic radiation includes, for example, amplified (e.g., laser) and non-amplified light, particularly in the UV and violet wavelength regions.
Many different elastomeric materials have been investigated for the preparation of the photopolymer plates. These include polyamide-based photopolymers (containing a polyamide as an essential component) which dissolves or swells in a washout solution (typically, water, alkaline aqueous solution, or an alcohol), polyvinyl alcohol-based photopolymers (containing polyvinyl alcohol as an essential component), polyester-based photopolymers (containing a low-molecular weight unsaturated polyester as an essential component), acrylic-based photopolymers (containing a low-molecular weight acrylic polymer as an essential component), butadiene copolymer-based photopolymers (containing a butadiene or isoprene/styrene copolymer as an essential component), and polyurethane-based photopolymers (containing polyurethane as an essential component), among others.
A slip film is a thin layer, which rests upon and protects the photopolymer from dust and increases its ease of handling. In a conventional (“analog”) plate making process, the slip film is transparent to UV light. The printer peels the cover sheet off the printing plate blank, and places a negative on top of the slip film layer. The plate and negative are then subjected to flood-exposure by UV light through the negative. The areas exposed to the light cure, or harden, and the unexposed areas are removed (developed) to create the relief image on the printing plate. Instead of a slip film, a matte layer may also be used to improve the ease of plate handling. The matte layer typically comprises fine particles (silica or similar) suspended in an aqueous binder solution. The matte layer is coated onto the photopolymer layer and then allowed to air dry. A negative is then placed on the matte layer for subsequent UV-flood exposure of the photocurable layer.
In a “digital” or “direct to plate” plate making process, a laser is guided by an image stored in an electronic data file and is used to create an in situ negative in a digital (i.e., laser ablatable) masking layer. The digital masking layer is typically a slip film which has been modified to include a radiation opaque material. Portions of the laser ablatable layer are ablated by exposing the masking layer to laser radiation at a selected wavelength and power of the laser. Examples of laser ablatable layers are disclosed for example, in U.S. Pat. No. 5,925,500 to Yang, et al., and U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, the subject matter of each of which is herein incorporated by reference in its entirety.
After imaging, the photosensitive printing element is developed to remove the unpolymerized portions of the layer of photopolymer material and reveal the crosslinked relief image in the cured photosensitive printing element. Typical methods of development include washing with various solvents or water, often with a brush. Other possibilities for development include the use of an air knife or heat plus a blotter (i.e. thermal development). The resulting surface has a relief pattern that reproduces the image to be printed. The relief pattern typically comprises a plurality of dots, and the shape of the dots and the depth of the relief, among other factors, affect the quality of the printed image. After the relief image is developed, the relief image printing element may be mounted on a printing press and printing commenced.
It is required that the printing plate have a relief image that has a chemical resistance to the ink that is used. It is also necessary that the physical and printing properties of the printing plate are stable and do not change during printing. Finally, in order to maintain high quality and clear printing during a run, it is highly desirable that a printing plate not pick up deposits of paper fibers and dried ink, which tend to fill in reverse areas of the plate and deposit at the edges of the printing areas of the plate. When plates pick up excessive deposits during printing, the printing press must be shut down periodically during the run to clean the plates, resulting in a loss of productivity.
Flexographic printing plates that are less likely to accumulate ink during use have been sought for many years, with limited success. The inherent need for the plate to accept ink on its relief surface often conflicts with attempts to limit its accumulation on other parts of the plate during use. Various attempts have been made to create clean relief image printing plates through modifications of the plate chemistry. However, none of these attempts has been very successful, often producing hazy plates that do not image well, or which failed to prevent the accumulation of ink.
Thus, there remains a need in the art for improved sheet photopolymer compositions that are solid at room temperature and that are capable of printing more cleanly and without picking up significant amounts of paper fibers, dust and ink during a print run. In addition, it is also desirable to improve the manufacturing process itself to produce relief image printing plates that are capable of printing cleanly and without picking up significant amounts of paper fibers, dust and ink during a print run.